This invention relates to presses for extracting water from a continuous traveling web and particularly to such a press section for extracting water from a newly formed web of paper in a papermaking machine. More particularly it relates to an extended nip press structure and an endless belt utilized in such press structure.
While the present invention relates to dewatering of a continuously running web of any material, it will be described herein with respect to the specific process of dewatering a web of paper. In the papermaking process, the web is formed by depositing the slurry of pulp fibers on a traveling wire. A large portion of the water is normally extracted from the web in the forming area by gravity or suction. The web then passes through what is known as a press section which normally would involve a series of nips of pairs of roll couples in which a substantial amount of the remaining water is squeezed out. The web will then pass on to a drying section which normally is composed of a series of heated drums to drive water off by vaporization. The web then finally passes to such finishing operations as calendering, coating, slitting, winding, et cetera.
The present invention relates specifically to a particular type of press section wherein the pressing operation in each unit is extended in time and thereby results in the extraction of significantly more water than in the heretofore nip of a roll couple. This extended nip pressing is accomplished by wrapping an endless belt about an arc of a rotating drum. The web is sandwiched between the endless belt and the drum and may have a traveling felt on one or both sides thereof for absorbing the water from the web. Additional pressure is provided to the arc of constant area by means of a pressure shoe located on the side of the belt opposite the drum.
The principles and advantages of extended nip pressing have been discussed in U.S. Pat. Nos. 3,798,121 and 3,853,698, both of which are assigned to the assignee of this invention. These principles and advantages, therefore, need not be discussed herein. The present invention, however, is related to an extended nip press of the type disclosed in U.S. Pat. No. 3,853,698 wherein a pressure shoe is located on the side of the belt opposite the drum to generate high pressing forces against the web. This is to be distinguished from the type disclosed in aforesaid U.S. Pat. No. 3,798,121 in which the pressure is provided by tension in one or more belts as they pass about the drum.
In the operation of such extended nip press sections having a pressure shoe, a problem has evolved wherein a bulge or bow forms ahead of the nip. The exact phenomenon which causes this bow or bulge is not fully understood. It is clear, however, that center portion of the endless belt in the area of the shoe is compressed, heated by the oil and friction and is otherwise worked differently than the rather wide edges of the belt. The bulge will sometimes be centered on the belt and at other times will be off to one lateral side of the belt. It will sometimes appear on the downstream side of the shoe on the laterally opposite side of the belt relative to a bulge on the upstream side of the belt. Experience thus far shows that the bulge is always confined in lateral directions to the shoe area.
Needless to say, this bulge in the belt is undesirable for many reasons, among which is the fact that it can cause wrinkling or creasing of the web. While the bulge can be eliminated by increasing the tension on the belt, this is not fully satisfactory since it causes increased loading on belts, shafts, bearings and drives. This in turn results in a decrease in the service life of such components and an increase in power consumption and down time.
The complexity of the operating conditions renders a solution to the problem evasive. Presently, pressure shoes having a 10 inch arc of contact and pressures of 600 pounds per square inch are utilized in experimental machines. This means that the belt is subjected to 6,000 pounds of normal force for every inch of width of the belt in the shoe area. Further, it is contemplated that pressures may be increased to 900 pounds per square inch or above and arcs of contact might be increased to as much as 20 inches or more. A 20 inch arc of contact and shoe pressures of 900 psi would result in 18,000 pounds of normal force for each inch of width of the belt in the shoe area.
Further, since the belt is in sliding contact with the shoe and under extremely high pressure, significant heat can be generated due to the sliding friction. The hydraulic fluid in the shoe is maintained at 140 degrees Fahrenheit (46 degrees Centigrade) to maintain the proper viscosity. With the heat caused by the sliding friction and hysteresis losses in the belt added to the heat from the oil, it is believed that belt temperatures may approach 200 degrees Fahrenheit (79 degrees Centrigrade).
In co-pending U.S. patent application Ser. No. 33,707 filed Apr. 26, 1979, by Dennis C. Cronin (assigned to the same assignee at this invention) it is suggested that longitudinally extending cords be provided only in the area of the belt which passes through the pressure shoe area. It is further noted in said co-pending application that by providing such longitudinals cord in the shoe area only, a substantial reduction in the tension required to eliminate the bulge is realized.
In my co-pending application U.S. Ser. No. 33,709 filed Apr. 26, 1979, relating to an extended nip press section for papermaking machines (assigned to the same assignee of this invention) it is proposed that the longitudinal reinforcing structure be comprised of at least a pair of layers of cords extending respectively at equal but opposite small angles with respect to the longitudinal direction of the belt. In my said co-pending application, it is noted that if the cord angle with respect to longitudinal direction is low and the modulus elasticity of the cords is sufficiently high, proper circumferential resistance can be accomplished and at the same time possible side to side variations and tensions throughout the shoe area can be balanced.
In accordance with the present invention, a transverse stiffening system is provided which resists bending which is required to form a bulge ahead of the shoe area. Specifically, one layer of wire cords will not significantly resist bending of the plane of the cords about an axis perpendicular to the cords. However, if two layers of wire cords are vulcanized together in a rubber matrix, they will effectively resist bending of the plane of the layer of cords about an axis perpendicular to the cords. In other words, the wires being substantially inextensible and incompressible will act in the nature of the webs in an I-beam and resist a bending moment.
Therefore, in accordance with the present invention, in addition to the circumferential tension resisting members in the belt, a transverse stiffening system comprising two layers of substantially inextensible and incompressible cords are provided in the belt structure.
Other objects, advantages and features will become more apparent with the disclosure of the principles of the invention and it will be apparent that equivalent structures and methods may be employed within the principles and scope of the invention in connection with the description of the preferred embodiment and the teaching of the principles in the specification, claims and drawings.